A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is positioned at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected or transmitted, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface, sometimes as a series of closely spaced adjacent two-dimensional lines and in other cases as a grid of source and receiver lines that are arranged to be at some other angle with respect to each other. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. (Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference.) General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 milliseconds and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface. Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profiles) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
The data in a 3D survey are amenable to viewing in a number of different ways. First, horizontal “constant time slices” may be taken extracted from a stacked or unstacked seismic volume by collecting all of the digital samples that occur at the same travel time. This operation results in a horizontal 2D plane of seismic data. By animating a series of 2D planes it is possible for the interpreter to pan through the volume, giving the impression that successive layers are being stripped away so that the information that lies underneath may be observed. Similarly, a vertical plane of seismic data may be taken at an arbitrary azimuth through the volume by collecting and displaying the seismic traces that lie along a particular line. This operation, in effect, extracts an individual 2D seismic line from within the 3D data volume.
Seismic data that have been properly acquired and processed can provide a wealth of information to the explorationist, one of the individuals within an oil company whose job it is to locate potential drilling sites. For example, a seismic profile gives the explorationist a broad view of the subsurface structure of the rock layers and often reveals important features associated with the entrapment and storage of hydrocarbons such as faults, folds, anticlines, unconformities, and sub-surface salt domes and reefs, among many others. During the computer processing of seismic data, estimates of subsurface rock velocities are routinely generated and near surface inhomogeneities are detected and displayed. In some cases, seismic data can be used to directly estimate rock porosity, water saturation, and hydrocarbon content. Less obviously, seismic waveform attributes such as phase, peak amplitude, peak-to-trough ratio, and a host of others, can often be empirically correlated with known hydrocarbon occurrences and that correlation applied to seismic data collected over new exploration targets.
In recent years there has been increasing interest in offshore hydrocarbon targets. This might be for many reasons, but certainly offshore tracts are attractive exploration targets because they offer some of the last relatively accessible unexplored targets. Of course, the seismic method has been key in this exploration effort.
Historically, marine seismic exploration has been performed using towed streamer arrays. This has proven to be an effective means of acquiring data over large areas. However, there are known problems with towed streamer surveys including, for example, the sometimes excessive amount of noise generated by the waves, boat engine, etc., can tend to obscure subtle (and/or deep) exploration targets.
As a consequence, in some instances it has proven to be advantageous to shift to an approach that utilizes receivers that have been placed on the ocean bottom. In this sort of seismic survey, ocean bottom seismometers (“OBS”) are typically placed at predetermined ocean bottom locations by either releasing them above the target location and letting them sink to the bottom or placing them on the ocean floor through the use of a remotely operated underwater vehicle (“ROV”). An OBS of the sort considered herein will preferably be a self-contained data-acquisition system that records seismic data generated by active sources such as airguns, as well as signals generated by passive seismic sources such as earthquakes, buoys, etc.
OBS units typically contain at least one pressure sensitive receiver (e.g., a hydrophone) and one vertical geophone. OBS receivers may also contain a four-component system, i.e., a hydrophone plus three orthogonal geophones. A four-component OBS has the advantage of being able to record shear waves, which would not be recordable by a conventional towed array survey. Finally, because they are stationary at the bottom of the ocean, surveys conducted using OBS-type equipment will tend to contain less noise than conventional marine surveys which can result in much improved subsurface imaging.
There are many advantages to OBS systems that may outweigh the additional cost (as compared with conventional/streamer recordings) in many circumstances. First, a four-component system has the potential of capturing both the P- and S-waves that were created by the seismic source. The converted-mode data recorded by such a survey can be used to improve reservoir characterization and imaging full-waveform seismic signals. Second, the resulting data is likely to be higher in bandwidth than would be obtained from a hydrophone-only survey. This has the potential to improve the illumination of complex targets (e.g., target proximate to or below salt domes). Additionally, greater sensitivity of the OBS systems means that data at greater azimuths may be captured and recorded. OBS systems also allow collection of regularly spaced seismic data in areas where there are obstructions that would impede the path of a seismic ship (e.g., offshore platforms, etc.). Finally, this approach allows receivers to be permanently deployed on the sea bed which increases the repeatability of the seismic collected during 4-D (i.e., time lapse) surveys.
However, OBS surveys are not without their problems. In addition to the increased cost (as compared with a conventional marine survey), certain unique problems arise when OBS equipment is employed.
One of the more vexing problems is that of determining the position and timing of each seismic unit. By way of explanation, after they are deployed the OBS units will typically be left on the ocean floor for some period of time during which time there may be limited (or no) communication between the OBS units and the source boat. The internal clock of an OBS device is typically synchronized with a standard clock before deployment and then again after recovery. Between deployment and recovery, the OBS receiver will rely on its internal clock which invariably tends to drift during the time it is in place. It is, thus, conventional to apply a deterministic time drift correction to the data after recovery to correct for accumulated inaccuracies in the clock timing. This correction, of course, may or may not be accurate. Although there are high accuracy clocks that would tend to reduce the magnitude of this problem (e.g., a very accurate clock with an accuracy of 10−10 seconds per day would provide sufficient accuracy for a typical ocean bottom survey of 30 days). However, such a solution is very costly.
If the free-fall mode of deployment is utilized, then the positions of those OBS units will not be precisely known. Instead, it is convention to determine the location of each receiver by reference to the first breaks of each shot. Of course, such a determination depends heavily on an accurate timing from the onboard clock. On the other hand, if a ROV is employed to plant the OBSs, the position of the units could be determined by a network of built-in acoustic modems. However, digital acoustic modems and highly accurate clocks are quite expensive and, additionally, tend to require much more power to operate that would a simple modem (used for limited communication with the surface boat) and a more conventional clock (e.g., one with an accuracy of about 10−7 seconds per day).
Heretofore, as is well known in the seismic processing and seismic interpretation arts, there has been a need for a better method of accurately determining OBS unit positions and timing. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.